Greene, J.W. 1860. XXI. Review of the American Bombidae, together with a description of several species heretofore undescribed, being a synopsis of this family of Hymenopterous insects thus far known to inhabit North America. Annals of the Lyceum of Natural History of New York 7: 168-176.

Justification:
According to our analysis, this western North American species has experienced rapid declines in relative abundance in recent years (Hatfield et al. 2014). The decade by decade relative abundance regression shows a gradual decline since the 1940s, and the relative abundance regression over just the past 50 years is highly significant (R-squared value of nearly 1; showing a continued steep decline). If we project the 50 year relative abundance regression into the future, it falls below the x-axis in the next 10 years. Notably, this species' regression mirrors that of B. occidentalis, a primary host. Both the past decline in relative abundance (90.11% over the past 10 years) and predicted future decline in relative abundance (based on 50-year regression) justify a Critically Endangered listing, using criterion A2be + A3b. Note that the range and persistence of this species have also declined, however, since some historic sites have not been re-sampled and since we only have records of this species in approximately six general localities for the current time period, we were not comfortable using those measures of decline. Based on the above calculations and trends, along with published reports of bumble bee decline and the assessors' best professional judgement, we recommend this species for the Critically Endangered Red List category at this time.

This species has a broad distribution centred in western North America and also including several scattered localities in the northeast. It occurs in the Mountain West from California and Colorado to Alaska, east to the Canadian Great Plains, with a disjunct subpopulation in Newfoundland (Williams et al. 2014).

Erroneous Records: Records of this species from Wisconsin on the Discover Life (2014) map for this species are in error (actually from Washington).

For a graph and map of relative abundance and range changes of this species over time, see the Supplementary Material.

This species is apparently declining in some parts of its range, presumably linked to declines of some of its hosts (Williams et al. 2014). We evaluated this species’ spatial distribution over time using a measure of change in the extent of occurrence (EOO; see Figure 3 in the Supplementary Material) and a measure of change in persistence (analytical methods described in Hatfield et al. 2014). We also assessed changes in the species’ relative abundance (see Figures 1 and 2 in the Supplementary Material), which we consider to be an index of abundance relevant to the taxon, as specified by the IUCN Red List Categories and Criteria (IUCN 2012). For all three calculations we divided the database into historical (1805-2001, N=128,572) and current (2002-2012, N=74,682) records. This timeframe was chosen to meet the IUCN criteria stipulation that species decline must have been observed over the longer of three generations or 10 years. Average decline for this species was calculated by averaging the change in abundance, persistence, and EOO. This analysis yielded the following results (see also the graph in Supplementary Material of relative abundance and map of change in EOO over time)

Current range size relative to historic range: 42.61%

Persistence in current range relative to historic occupancy: 15.95%

Current relative abundance relative to historic values: 9.89%

Average decline: 77.18%

This cuckoo bumble bee has steeply declined in relative abundance, particularly over the past 50 years (Hatfield et al. 2014; see Figure 2 in the Supplementary Material). The decade by decade relative abundance regression shows a gradual decline since the 1940s, and the relative abundance regression over just the past 50 years is highly significant (R-squared value of nearly 1; showing a continued steep decline). If we project the 50 year relative abundance regression into the future, it falls below the x-axis in the next 10 years. Notably, this species' regression mirrors that of B. occidentalis. Note that although the range and persistence of this species have also declined dramatically, we only have records of this species in six general locations for the current time period, and since some historic sites have not been revisited and we lack population level data, we are not relying on these measures in this assessment.

For a graph and map of relative abundance and range changes of this species over time, see the Supplementary Material.

Bombus suckleyi is a cuckoo bee, a term used for a specialized lineage of bumble bees (subgenus Psithyrus) that has lost the ability to collect pollen and to rear their brood. As such, these bees do not found their own nests, but instead, usurp the colonies and worker forces of other bumble bee species. To do this, a mated female enters the nest of another bumble bee species, kills or subdues the queen of that colony, and forcibly (using pheromones and/or physical attacks) "enslaves" the workers of that colony to feed her and her developing young. Since all of the resulting cuckoo bee offspring are reproductive (not workers), they leave the colony to mate, and the mated females seek out other nests to attack. Males of this species patrol circuits in search of mates. Before finding and invading a host colony, females feed directly from flowers. The adult food plants of this species include "Aster", Chrysothamnus, Cirsium, and Solidago (Williams et al. 2014).

Cuckoo bee nesting occurs exclusively in the nests of other bees. They often attack a broad range of host species, but some species specialize in attacking the members of just one species or subgenus. Bombus suckleyi has been recorded in nests of bumble bees in six different subgenera, but the most common association were with the subgenera Pyrobombus and Bombus, and the only nests in which B. suckleyiadults were produced were those of B. occidentalis (reviewed in Thorp et al. 1983).As such, B. suckleyi has been documented breeding as a parasite of colonies of Bombus occidentalis, and has been recorded as present in the colonies of B. terricola,B. rufocinctus, B. fervidus, B. nevadensis, and B. appositus (Williams et al. 2014).

The recent steep declines in relative abundance exhibited by this cuckoo bee are probably due to indirect threats (e.g., disease, decline in habitat) resulting in loss of hosts (Hatfield et al. 2014, Williams et al. 2014). This species is an obligate nest parasite of other bumble bees. Some of these bees, particularly B. occidentalis and B. terricola, are also experiencing dramatic declines (Evans et al. 2008), which is likely driving the declines of this species. Additional direct threats that may be impacting this species include pesticide use, habitat loss, pathogens from managed pollinators, competition with non-native bees, and climate change (reviewed in Goulson 2010, Williams et al. 2009, Williams and Osborne 2009, Fürst et al. 2014, Cameron et al. 2011, Hatfield et al. 2012). Reduced genetic diversity resulting from any of these threats can be particularly concerning for bumble bees, since their method of sex-determination can be disrupted by inbreeding (Goulson 2010, Hatfield et al. 2012). Note that Cordes et al. (2012) found this species exhibited high prevalence of the microsporidium Nosema bombi (25%), although the sample size was low (four individuals).

Specific conservation and research needs for this species have not been identified. Due to the inherent vulnerability of many bumble bee species and importance of supporting wild bee populations for pollination services, the following general conservation practices are recommended: